1. Field of the Invention
This invention relates to light-reflecting molded or extruded articles having a reflectivity high enough to enhance the efficiency of illumination sources, and to methods of making the same. More specifically, the present invention relates to light-reflecting articles made of synthetic organic polymers (xe2x80x9cplasticsxe2x80x9d) whose structural strength, dimensional stability and other properties make them suitable for use as light-reflecting articles such as light-reflecting enclosures for use in conjunction with light sources. The plastic articles of the invention have a total reflectivity of at least about 93 percent, preferably at least about 94 percent, more preferably at least about 95 percent and most preferably at least about 96 percent, which is attained without the necessity of using reflective films or metal coatings.
2. Related Art
U.S. Pat. No. 5,589,524 to Y. Ogita et al, issued Dec. 31, 1996, provides a polyphenylene sulfide resin composition said to be useful for making a light-reflective molded article comprising from 40 to 60 percent by weight of a polyphenylene sulfide resin containing from 30 to 50 percent by weight calcium carbonate having an average particle size of from 1 to 7 microns, and from 8 to 25 percent by weight of mica having an average particle size of from 1 to 10 microns. (See the Abstract.) The use of other fillers in the composition is disclosed at column 4, lines 38-48, xe2x80x9cso long as the effect of the present invention is not impaired.xe2x80x9d A list of such other fillers includes antimony trioxide, zinc oxide, and titanium oxides, among numerous other examples.
U.S. Pat. No. 4,132,563 to Burke, Jr. et al, issued Jan. 2, 1979, discloses an intraleucospheruloid/organic color pigment composition and a process for producing the same. As set forth in the Abstract of the disclosure, the intraleucospheruloid/organic color pigment composition consists essentially of spheruloids of essentially transparent polymer material having primary particles of an average size not exceeding 4 microns in diameter, and which have embedded therein a particulate leuco pigment composition. The latter consists essentially of inorganic opaque white and/or transparent white pigment material having a different refractive index from that of the polymer, and primary particles of an average size not exceeding 0.2 microns in diameter, plus an organic pigment material. The organic pigment material is said to consist of organic color pigment compositions having primary particles of an average size of less than 0.2 microns in diameter. As disclosed starting at column 1, line 38 of this Patent, the pigment compositions of the invention are said to have a very high degree of light stability regardless of the use to which they are put, and can also provide improved physical characteristics such as storage stability, ease of handling and incorporation into vehicles such as paints and thermoplastic materials.
U.S. Pat. No. 5,672,409 to K. Miyakawa et al, issued Sep. 30, 1997 and, as shown in the Abstract, discloses a white polyester film reflector used in a surface light source. The white polyester film has fine voids formed therein and its apparent specific gravity is in the range of 0.5 to 1.2. The reflectance of the polyester film is said to be increased by the presence of the fine voids. One disclosed use for the reflector is in a surface light source having a side light system to provide, for example, a bright scope of a liquid crystal display.
U.S. Pat. No. 5,428,105 to F. J. McGarry, issued Jun. 27, 1995 and is based on a division of the application which matured into U.S. Pat. No. 5,376,721 on Dec. 27, 1994. This patent discloses a curable polyester molding composition incorporating an elastomeric low-profile additive dispersed in voided rubbery particle form throughout the composition. A compatibilizing agent is used in association with the additive to form a molded thermoset article having very smooth light-reflective class A surfaces and other qualities as enumerated, for example, in the Abstract.
A technical brochure bearing a 1997 copyright notice of E.I. Du Pont de Nemours and Company is entitled xe2x80x9cTi-Puret Titanium Dioxidexe2x80x9d. This brochure describes optical and other properties of titanium dioxide and discusses the effect of particle size of the titania on the color tint of transmitted light. For example, at page 1 of the brochure, it is stated that the transmitted appearance of tints or whites which contain small particle size titania grades are yellower than similar products containing a larger particle size titania. In Appendix C of the brochure, there is a discussion of the effect of particle size of the titania additive on the appearance and performance of the products in which it is used. Table I gives indices of refraction for some white pigments, including rutile and anatase titania, and some common plastics.
In accordance with the present invention, there is provided a light-reflecting article which comprises a thermoplastic or thermoset polymer matrix having dispersed therein a flame-retardant material and dispersed particles of a reflective filler. The reflective filler, e.g., one or more of rutile or anatase titania, zinc sulfide or strontium titanate, comprises a dielectric material having a particle size and reflective properties which enhance total reflectivity of the composite. The filler further has an index of refraction which differs from the effective index of refraction of the other components of the composite to a degree sufficient to enhance the total reflectivity of the composite. The filler and flame-retardant material are sufficiently dispersed and present in amounts such that the article has an average total reflectivity as measured from a flat sample of at least about 93 percent, preferably at least 94 percent, more preferably, at least 95 percent, over the range of visible light.
In one aspect of the present invention, the article has a flame retardance equivalent to a LT-94 rating (defined below) of at least V-1, i.e., of V-1 or V-0. In another aspect, the flame-retardant material comprises (i) particles of a metal oxide synergist, e.g., one or more of antimony trioxide, antimony pentoxide and sodium antimonate, and (ii) a halogenated organic compound quencher, e.g., a brominated organic compound such as bis( tribromo-phenoxy)ethane or an organic phosphate or borate. The flame-retardant material is present in an amount sufficient to reduce flammability of the article relative to an otherwise identical article lacking the flame-retardant material, e.g., the flammability is reduced to a level equivalent to a UL-94 rating of V-1 or V-0. Further, the filler and the flame-retardant material are present in amounts such that the article has a reflectivity of at least about 93 percent for light in a wavelength range of from about 400 to 700 nanometers, measured as total reflectivity (specular plus diffuse) from a flat sample.
A further aspect of the present invention provides that the particles of filler have a size range wherein: when the filler comprises titania, at least about 70 percent by weight of the titania particles have a diameter of from about 0.12 to 0.44 microns; when the filler comprises zinc sulfide, at least about 70 percent by weight of the zinc sulfide particles have a diameter of from about 0.17 to 0.65 microns; and when the filler comprises strontium titanate, at least about 70 percent by weight of the strontium titanate particles have a diameter of from about 0.16 to 0.62 microns.
Another aspect of the present invention provides for the inclusion of a brightener selected from the group consisting of fluorescent materials which absorb radiant energy in the ultraviolet region of the electromagnetic spectrum (wavelengths 300 nanometers to 400 nanometers) and emit this energy as visible light in the blue region of the spectrum (wavelengths from 400 nanometers to 470 nanometers). Examples of such materials include Eastobrite(copyright) OB-1 and OB-3 produced by Eastman Chemical Corp. of Kingsport, Tenn., Uvitexo-(copyright)-OB, and Uvitex(copyright)-MD produced by Ciba Specialty Chemicals of Basel, Switzerland, and Leucopure(copyright) EGM, Hostalux KCB, and Hostalux KSB from Clariant of Muttenz, Switzerland.
The present invention also provides that the article as aforesaid may comprise a light-reflecting enclosure, the enclosure having at least one aperture formed therein and being dimensioned and configured to receive therein a light source disposed within the enclosure to transmit light through the aperture.
In accordance with the present invention the polymer matrix may be any substantially clear or white thermoplastic or thermosetting polymer, such as those chosen from the group consisting of polyacrylonitrile, polyesters, epoxies, polyurethanes, acrylonitrile-butadiene-styrene terpolymer, poly(methylpentene), polypropylene, polystyrene, polycarbonate, polymethacrylate, polyacrylate, poly(vinyl chloride), chlorinated polyethylene, and substituted derivatives, copolymers, blends and alloys of any of the foregoing. In a related preferred aspect of the invention, the polymer matrix comprises a blend of the acrylonitrile-butadiene-styrene copolymer with polyvinyl chloride polymer, or a blend of the acrylonitrile-butadiene-styrene terpolymer with a polycarbonate polymer. For example, the polymer matrix may comprise from about 10 to 50 weight percent of the acrylonitrile-butadiene-styrene terpolymer and from about 40 to 90 weight percent of the polycarbonate polymer.
In accordance with another major aspect of the present invention, there is provided a light-reflecting article comprising a thermoplastic or thermosetting polymer matrix having a refractive index of n1, the polymer having dispersed therein particles of a reflective filler having a refractive index of n2. The particles of filler have a size range wherein at least about 60 percent by weight, e.g., at least about 70 percent by weight, of the particles have a diameter of from about D to 4D, wherein D=xcexxcex4o/xcfx80n1xcex4, in which xcexo is the mean free-space wavelength of the light range being reflected and xcex4=|n2xe2x88x92n1|. The filler is present in an amount such that the article has a reflectivity of at least about 93 percent, e.g., at least about 94 percent, or at least about 95 percent, for light in a wavelength range of from about 400 to 700 nanometers, measured as total reflectivity from a flat sample.
Other aspects of the present invention are set forth in the following description and the appended drawings.